Cycles, trends and residual variation in the Iberian sardine (Sardina pilchardus) recruitment series and their relationship with the environment.

Abstract

Recruitment variability is an important component of the dynamics of Ibe-rian sardine (Sardine pilchardus). Since 2006, poor recruitment has led to a fall in stock biomass, the latest in a series of such crises for sardine fisheries. Understanding the mechanism behind recruitment fluctuations has been the objective of many previous studies, which proposed various relationships between recruitment and environmen-tal variables. However, such studies face several analytical challenges including short time series and autocorrelated data. We carry out new analysis of empirical relation-ships with environmental series using statistical methods designed to cope with these issues, including dynamic factor analysis, generalised additive models and mixed models. Our results identify relationships between recruitment and global (number of sunspots), regional (NAOAutumn) and local (winter wind strength, SST and upwel-ling) environmental variables. Separating these series into trend and noise components permitted further investigation of the nature of the relationships. Whereas the other three environmental variables were related to the trend in recruitment, SST was re-lated to residual variation around the trend, providing stronger evidence for a causal link, possible mechanisms for which are discussed. After removal of trend and cyclic components, recruitment also shows a weak relationship with the previous year´s spawning stock biomass.